Orientation, balance, position, and movement of a body can be determined by the brain through a combination of signals received from various parts of anatomy, including eyes, ears, and muscles. For example, the vestibular system, in most mammals, is the sensory system that predominantly contributes sensory information related to balance and spatial orientation. The vestibular system of a subject is found in the inner ear of the subject, as shown in FIG. 1A, in a system of interconnected compartments forming the vestibular labyrinth.
FIG. 1A illustrates a portion of the anatomy of a subject 100, showing the vestibular system with respect to an external ear 110, portions of a skull 114, and bony portion of an ear 116, an ear canal 111, an ear drum 112, and the bones of a middle ear 113. The vestibular system includes semicircular canals 122, 124, and 126, and otolith organs 128 and 130, housed within a vestibule 121 in the bony labyrinth of the inner ear, and is continuous with a cochlea 120. FIG. 1B provides a more detailed illustration of the vestibular system shown in FIG. 1A, depicting the vestibule 121 to include a utricle 128 and a saccule 130.
The three semicircular canals 122, 124, and 126 are each oriented in a plane along one of three directions in which the head can rotate or move and detect motion in that direction, the directions being nodding up-down, shaking left-right, and tilting left-right. The otolith organs within the vestibule of the inner ear 121 detect gravitational forces and acceleration in the forward and backward directions. The otolith organs include the utricle 128 that detects movements in the horizontal plane and the saccule 130 that detects movements in the vertical plane. The semicircular canals 122, 124, and 126, and the otolith organs 128 and 130 are filled with endolymph, a fluid that moves with the movement of the head or body.
The movement of endolymph in the vestibular system of the inner ear can be sensed by nerve cells with hair bundles to determine movement and orientation of the head. Portions called ampula in the semicircular canals and macula in the otolith organs include hair cells, which function as the sensory receptors of the vestibular system and include hair bundles or stereocilia that detect and transduce movement of the endolymph into signals of body movement and report the signals to the brain. The otolith organs also include a layer of crystals of calcium carbonate called otoconia or otoliths that shift in response to changes in acceleration (e.g., changes in motion or orientation with respect to gravity) leading to movement in the layers below the otoconia and the movement of hair bundles. Additionally, otoliths sink in the direction of gravity and pull on bundles of hair cells to aid in distinguishing directions, e.g., up from down.
FIGS. 2A and 2B provide detailed views of the anatomy of the macula in the otolith organs (e.g., the utricle 128 and the saccule 130 shown in FIG. 1B) and the sensory receptors, in an upright state and in a state of movement, respectively. FIG. 2A shows the macula including an otolithic membrane 132 and a cellular layer including hair cells 134 and supporting cells 136. The hair cells 134 include hair like projections or stereocilia 132 that extend into one or more gelatinous layers. The organization of the macula also includes a layer of otoconia or otoliths 138 that shift in response to movement in the endolymph and/or to acceleration of the body. FIG. 2A shows the hair cells 134 and the otoliths 138 in an upright configuration, and FIG. 2B shows the hair cells 134 and the otoliths 138 in a displaced or angled configuration when a directional force 140 (e.g. gravity) acts on the otoliths 138. Similarly, movement of the endolymph within the semicircular canals 122, 124, and 126, can result in movement of the hair cells within the ampula of the semicircular canals (not shown) perceiving and signaling relative movement of the body and/or head (e.g., angular acceleration of the head).
In addition to signals from the vestibular system, horizontal and vertical visual patterns received by the eyes can affect perception of orientation, balance, and position; and differential strain on opposing neck muscles can affect perception of head position and orientation. When signals from these sources do not match, an individual can develop motion sickness, experience vertigo, dizziness, vestibular migraines, unconsciousness, or other conditions. Unmatched orientation, balance, position and movement signals can be the result of extreme or unfamiliar movement during, for example, travel in cars, trains, airplanes, and other modes of transportation. Unmatched signals may also result from simulated perceived movement during, for example, three dimensional (3D) movies, 3D video games, and virtual reality devices. Therefore, it can be desirable to have a device for treating various vestibular conditions that may result from unmatched signals being received from a subject's vestibular system, eyes, or other anatomy.